Skeletal muscle regeneration was studied in polymyopathic hamsters of the strain BIO 8262 by autoradiography. Flash labeling with 3H-thymidine revealed high labeling indices of mononuclear cells in areas with muscle cell necroses and low labeling of "interstitial cells". However, no labeled nuclei were present in myotubes. Three days after multiple injections of the radioisotope, labeled nuclei in the myotubes were present. The mean labeling intensity of these nuclei was slightly elevated on the 15th day. On the third day after multiple 3H-thymidine injections nearly all mononuclear cells were labeled; but their labeling indices slightly decreased until the 15th day. From the decrease of labeling intensity, a high turnover rate of these cells may be assumed. Labeled myotubes appeared in the neighborhood of mononuclear cells as well as in areas without pronounced round cell infiltrations. From labeled myotubes in areas with round cell infiltrations it may be concluded that they arose by fusion of rapidly proliferating satellite cells, which may be constituents of the mononuclear cell population. Labeled myotubes that appeared outside areas of mononuclear cell infiltrations may have been caused by fusion of slow proliferating satellite cells. Despite the fact of a high regeneration capacity of muscle in hamsters of the strain BIO 8262, this process is not sufficient to completely restore the muscle necroses.